In optical communications networks, optical transceiver modules are used to transmit and receive optical signals over optical fibers. On the transmit side of the transceiver module, a laser generates amplitude modulated optical signals that represent data, which are then transmitted over an optical fiber coupled to the transceiver module. Various types of semiconductor lasers are typically used for this purpose, including, for example, Vertical Cavity Surface Emitting Lasers (VCSELs) and edge emitting lasers, which may be further divided into subtypes that include Fabry Perot (FP) and Distributed Feedback (DFB) lasers.
On the receive side of the transceiver module, an optics system of the module directs light propagating out of the end of an optical fiber onto an optical detector, which converts the optical energy into electrical energy. The optical detector is typically a semiconductor photodiode device, such as a PIN photodiode, for example. Optical transceiver modules typically include one or more laser diodes on the transmit side for transmitting multiple data signals and one or more photodiodes on the receive side for receiving multiple data signals.
Small form factor optical subassemblies (OSAs) known as transistor outline (TO)-cans are used to hermetically package certain components of optical transceiver modules, such as, for example, laser diodes and photodiodes. In a transmitter OSA (TOSA) TO-can, a die attach process that typically uses epoxy or gold-tin eutectic alloys is performed to attach the laser diode integrated circuit (IC) die to a substrate of a submount assembly of the TOSA. Other components such as discrete electrical components and optical elements are typically also attached to the substrate. After the laser diode IC die and other components have been attached to the substrate, the leads of the die are wire bonded to conductors formed in the submount assembly substrate. These conductors are then wire bonded to pins of the TOSA, which will be used to connect electrical components external to the TOSA to the electrical components inside of the TOSA.
After the wire bonding process has been performed, a cap or lid having a transparent window in it is hermetically sealed to the TO-can header in such a way that the lid encompasses the TOSA components in a hermetically-sealed environment. The pins of the TOSA are then electrically coupled by soldering to conductive traces of a printed circuit board (PCB) that is external to the TOSA. Various other electrical components are mounted on the PCB and electrically connected to the traces of the PCB to provide electrical connections between the electrical components on the PCB and the pins of the TOSA.
Typical low-cost TOSA TO-cans have four or five pins on the submount assembly substrate for power, ground, data, laser output power monitoring, and temperature monitoring. The output power of the laser diode is typically monitored by a discrete photodiode that is placed inside of the TO-can and electrically connected to conductors of the submount assembly substrate. In these types of transceiver modules, the temperature of the laser diode is typically monitored by a sensor that is external to the TOSA. No components are included inside of the TOSA for performing temperature control or temperature monitoring because these types of components require that the TOSA have one or more additional pins. These types of TO-cans also do not include components for performing temperature control or temperature monitoring because of space and power constraints of the TOSA.
In more expensive transceiver modules, the TOSAs have a greater numbers of pins. In these types of transceiver modules, components for laser temperature monitoring and/or temperature control are often included inside of the TOSA. In these types of transceiver modules, the TOSA may include a thermo electric cooler, a resistive heater element or a thermistor for measuring the temperature of the laser diode. Consequently, extra pins are provided on the TOSA for sending control signals to and/or receiving output signals from these components. The operating wavelength of the laser diode can be adjusted to a desired wavelength and maintained at that wavelength by adjusting the temperature of the laser diode. Also, as a laser diode ages, the wavelength of light it produces at a given temperature tends to shift. By monitoring and adjusting the temperature of the laser diode as needed, the operating wavelength of the laser can be maintained at a desired wavelength.
These types of TOSAs sometimes also include an additional photodiode for monitoring the wavelength being produced by the laser diode. TOSAs of this type also sometimes also include a discrete electrostatic discharge (ESD) diode that is wire bonded to the conductors of the submount assembly substrate in parallel with the laser diode to protect the laser diode from ESD events. Alternatively, the ESD diode is sometimes clipped to pins of the TOSA. It would be desirable for a variety of reasons to provide low-cost TO-can OSAs with functionality of the type described above for controlling and/or monitoring the temperature of the laser diode. However, as described above, this additional functionality normally cannot be provided in low-cost OSAs without increasing the OSA pin count and the size of the OSA. Accordingly, a need exists for a low-cost TO-can OSA having functionality for controlling and/or monitoring the temperature of the laser diode, and which does not require an increase in pin count or size of the OSA.